1. Field of the Invention
The present invention relates to solder materials used in microelectronic device packaging. In particular, the present invention relates to altering the coefficient of thermal expansion of solder used in mechanical, thermal, and/or electrical contacts in microelectronic device packages.
2. State of the Art
A variety of materials are used in the microelectronics industry for a forming components and attaching components together. One highly versatile material is solder, primarily alloys of lead and tin. Solder may be used for mechanical, electrical, and/or thermal attachment of components.
FIG. 4 illustrates an exemplary flip-chip attachment of a microelectronic device 202 to a substrate 204, such as an FR-4 substrate. The microelectronic device 202 is mechanically and electrically attached to the substrate 204 through a plurality of solder balls 206 which extend between discrete contact pads 208 on an active surface 212 of the microelectronic device 202 and contact pads 214 on the substrate 204. A heat dissipation device (shown as a heat slug 216 is mechanically attached to a back surface 218 of the microelectronic device 202 with a layer of solder 222. The solder layer 222 allows thermal conduction of heat from the microelectronic device 202 to the heat slug 216. The heat from the microelectronic device 202 is dissipated from the heat slug 216 to the ambient environment. Thus, solder is used for both the mechanical and electrical attachment of the microelectronic device 202 to the substrate 204 and the mechanical and thermal attachment of the heat slug 216 to the microelectronic device 202.
One problem which must be addressed in the connection of various different types of materials (i.e., microelectronic devices, substrates, heat slugs, etc.) is the coefficient of thermal expansion (CTE) for each material. The CTE is a measurement of the expansion and contraction of each material during heating and cooling cycles, respectively. These heating and cooling cycles occur during the operation of the microelectronic device 202 and during power up and power down of the microelectronic device 202.
The attachment of a heat dissipation device to a microelectronic device is particularly problematical. The material of the heat dissipation device (e.g., heat slug 216) generally comprises a metal, while the material of the microelectric device 202 generally comprises a silicon material. The CTE difference between a metal and a silicon material is considerable. The CTE difference between the heat slug 216 and the microelectronic device 202 results in stresses that may degrade the performance of the microelectronic device 202 through warpage and may result in premature device failure, such as by the cracking of the microelectronic device 202.
The microelectronics industry has addressed the CTE mismatch problem with a variety of solutions, such as altering the CTE of the materials used. For example, in heat dissipation devices may be made from aluminum filled with silicon carbide ceramic particles. The silicon carbide ceramic filled aluminum has high heat transfer and has a lessened CTE compared to pure aluminum. Such silicon carbide carbide ceramic filled aluminum products are available from Lanxide Electronic Components, Inc., Newark, Del., USA.
Although the CTE mismatch problem has been or is being continuously addressed, another CTE problem has arisen as a result of industry""s continuous generation of smaller and smaller microelectronic devices. The smaller microelectronic devices result in the CTE of the attachment materials, such as solder, having an effect on the microelectronic package.
Unfortunately, the solders presently used generally have a higher CTE than the materials they join (i.e., they expand and contract at a faster rate than the surrounding material when heated and cooled, respectively). For example, in the attachment of the heat slug 216 to the microelectronic device 202, the CTE mismatch of solder may result in the solder layer 222 becoming detached from the either the heat slug 216 or the microelectronic device 202. This detachment may reduce the efficiency of the thermal contact between the heat slug 216 and the microelectronic device 202, which can result in the premature failing of the microelectronic device 202 due to thermal degradation. With regard to the solder balls 206, the CTE mismatch of the solder in the solder balls 206 may result in one or more of the solder balls 206 becoming detached from the substrate contact pad 214 or the microelectronic device contact pad 208, either of which may result in a failure of the microelectronic device 202.
Therefore, it would be advantageous to develop a solder which has a CTE comparable to the components to which the solder is in electrical, mechanical, and/or thermal contact.
The present invention relates to altering the coefficient of thermal expansion of solder used in thermal and/or electrical contacts in microelectronic packages. Specifically, the present invention relates to a filled solder material comprising a solder material having a plurality of coated filler particles disposed therein.